The present invention generally concerns a mechanical device displaying cycles of discrete values of a variable on a scale of N fields, the variable being able to take either N successive ordinary values in an ordinary cycle, or at least N+1 values, including said N ordinary values and at least one additional value, in an extraordinary cycle.
This device applies particularly to the display of a lunar month in a lunisolar calendar mechanism, particularly in a timepiece. The number or the name of the current lunar month is displayed from a lunisolar mechanism such as for example the conventional Chinese calendar. A lunisolar calendar is based above all on the lunations, whose mean length is not equal to an integer number of days. Known mechanisms for displaying variables of the Julian calendar or another solar calendar cannot therefore be used for this purpose.
The Chinese calendar is still used nowadays to fix the date of certain holydays and for Chinese astrology. It is of the lunisolar type, in that it comprises lunar months that correspond to lunations, whereas Chinese years have a variable number of months in order to remain as close as possible to tropical years, i.e. to the apparent movement of the sun over the ecliptic. This calendar has a cycle of nineteen years, called the Chang cycle, which comprises almost integer numbers of lunations (235), tropical years (19) and Chinese years (19), and the origin of which is fixed so as to satisfy the historical condition fixing the Chinese New Year at the second new moon that follows the winter solstice, with rare exceptions. Each of these periods of nineteen Chinese years includes twelve ordinary years of twelve lunar months and seven bissextile years of thirteen lunar months. If one numbers the years in the Chang cycle, the bissextile years typically bear the numbers 1, 4, 7, 10, 12, 15 and 18. The additional lunar month of the bissextile years is inserted between two of the ordinary months, at a non-cyclical position which depends on astronomical data and which thus varies from one bissextile year to another. It is given the same number as the preceding month, so that the following lunar months keep the same numbers as in an ordinary year. Depending upon the time of the new moon of each New Year concerned, an ordinary year of the Chinese calendar can comprise 353, 354 or 355 days, whereas a bissextile year can comprise 383, 384 or 385 days.
For more data concerning the Chinese calendar, the reader can refer to the work by Nachum DERSHOWITZ and Edward M. REINGOLD, Calendrical Calculations, Cambridge University Press, 1997; and to the publications by Helmer ASLAKSEN: the Mathematics of the Chinese Calendar, 13 May 2004, and Bissextile Months.nb, Mathematica package, 1999, available on the site www.math.nus.edu.sg.
It will be noted that other lunisolar calendars exist to which the present invention could apply, for example the ancient Greek calendar and the Jewish calendar, in which the bissextile years also have a cycle of nineteen years, called the Méton cycle, which is very similar to the Chang cycle. U.S. Pat. No. 4,055,749 discloses an electronic apparatus able to display the Jewish calendar.